This invention relates to a new process for the continuous production of organic polyisocyanates, more particularly diisocyanates, by thermal decomposition of the carbamic acid esters on which they are based.
It has been known for many years that N-substituted urethanes can be split into isocyanates and alcohol [H. Schiff, Ber. Dtsch. Chem. Ges. 3, 649 (1870); A. W. Hoffmann, Ber. Dtsch. Chem. Ges. 3, 653 (1870)].
Some time ago, it was found that the decomposition reaction can be carried out with various advantages in the presence of high-boiling solvents for the carbamic acid esters. Thus, U.S. Pat. No. 3,919,278, for example, describes a process for the production of aromatic polyisocyanates in a decomposition reactor surmounted by a distillation column using auxiliary solvents of the type in question and also a carrier gas to improve the removal of the decomposition products formed during the decomposition reaction from the reaction mixture. U.S. Pat. No. 3,962,302 describes a similar process. Both processes result in relatively poor yields which is presumably attributable to the fact that the polyisocyanates formed as decomposition products are difficult to separate from the decomposition medium. It is presumably for this reason that, according to DE-OS 2 530 001, the decomposition medium is subjected to separate working up to recover the diisocyanate present therein.
In addition to these processes carried out in the absence of catalysts, there are also processes in which the yield is supposed to be increased by the use of various catalysts. Relevant publications are, for example, U.S. Pat. No. 3,919,279, DE-OS 2,635,490, DE-OS 2,942,543 or EP-A-0,323,514. It is obvious that the use of catalysts involves certain disadvantages compared with a catalyst-free process. For example, traces of catalyst can never be completely prevented from entering the end products and undesirably affecting their properties. In addition, the working up of the auxiliary solvent is complicated by the presence of the catalysts.
Accordingly, the problem addressed by the present invention was to provide a new process for the production of organic polyisocyanates, more particularly diisocyanates, by decomposition of the corresponding carbamic acid esters which would readily enable the diisocyanates to be continuously produced in high yields in the absence of catalysts.
This problem has been solved by the provision of the process according to the invention which is described in detail hereinafter.